Half-hours with the Telescope eBook

But interesting as these views may be, it was not
for such discoveries as these that astronomers examined
the surface of the moon. The examination of mere
peculiarities of physical condition is, after all,
but barren labour, if it lead to no discovery of physical
variation. The principal charm of astronomy,
as indeed of all observational science, lies in the
study of change—­of progress, development,
and decay, and specially of systematic variations
taking place in regularly-recurring cycles. And
it is in this relation that the moon has been so disappointing
an object of astronomical observation. For two
centuries and a half her face has been scanned with
the closest possible scrutiny; her features have been
portrayed in elaborate maps; many an astronomer has
given a large portion of his life to the work of examining
craters, plains, mountains, and valleys, for the signs
of change; but until lately no certain evidence—­or
rather, no evidence save of the most doubtful character—­has
been afforded that the moon is other than “a
dead and useless waste of extinct volcanoes.”
Whether the examination of the remarkable spot called
Linne—­where lately signs were supposed to
have been seen of a process of volcanic eruption—­will
prove an exception to this rule, remains to be seen.
The evidence seems to me strongly to favour the supposition
of a change of some sort having taken place in this
neighbourhood.

The sort of scrutiny required for the discovery of
changes, or for the determination of their extent,
is far too close and laborious to be attractive to
the general observer. Yet the kind of observation
which avails best for the purpose is perhaps also
the most interesting which he can apply to the lunar
details. The peculiarities presented by a spot
upon the moon are to be observed from hour to hour
(or from day to day, according to the size of the
spot) as the sun’s light gradually sweeps across
it, until the spot is fully lighted; then as the moon
wanes and the sun’s light gradually passes from
the spot, the series of observations is to be renewed.
A comparison of them is likely—­especially
if the observer is a good artist and has executed
several faithful delineations of the region under
observation, to throw much light upon the real contour
of the moon’s surface at this point.

In the two lunar views in Plate 7 some of the peculiarities
I have described are illustrated. But the patient
observer will easily be able to construct for himself
a set of interesting views of different regions.

It may be noticed that for observation of the waning
moon there is no occasion to wait for those hours
in which only the waning moon is visible during
the night. Of course for the observation of
a particular region under a particular illumination,
the observer has no choice as to hour. But for
generally interesting observations of the waning moon
he can wait till morning and observe by daylight.
The moon is, of course, very easily found by the unaided
eye (in the day time) when not very near to the sun;
and the methods described in Chapter V. will enable
the observer to find the moon when she is so near to
the sun as to present the narrowest possible sickle
of light.